The present invention relates to a method for the selective oxidation of carbon monoxide (CO) in gas streams comprising carbon monoxide, hydrogen and oxygen. More specifically the invention relates to the method of operation of fixed-bed catalytic reactors operating adiabatically to reduce the degree of the reverse water gas shift reaction that can occur in selective oxidation reactors.
Hydrogen is becoming an increasingly desired fuel. One method of obtaining hydrogen is to release it from hydrocarbons. This approach suffers from the simultaneous production of carbon monoxide. Hydrogen containing carbon monoxide impairs the performance of many systems such as ammonia synthesis reactors and low temperature fuel cells. It is therefore desirable to have mechanisms to remove carbon monoxide from hydrogen. One method of accomplishing this removal is the selective oxidation of the carbon monoxide using a fixed-bed catalytic reactor.
Catalytic reactors of the fixed-bed type to selectively oxidize carbon monoxide are well known in the art. It is also well known that these fixed-bed reactors when operated for the selective oxidation of CO under varying flow conditions, such as reduced load conditions in fuel processing for fuel cell applications, can actually produce carbon monoxide via the reverse water gas shift reaction, the reaction occurring when the oxygen concentration within the bed is depleted below a minimum threshold value. Thus if oxygen is consumed to this minimal threshold value before the hydrogen containing gas stream exits the catalyst bed, carbon monoxide is reformed in the oxygen depleted zone of the reactor. Addition of additional oxygen into the oxygen depleted zone of the reactor, as proposed in U.S. Pat. No. 5,811,692, prevents the reverse water gas shift reaction but at the expense of additional hydrogen consumption and added operational complexity.
It has now been found that the reverse water gas shift reaction within a fixed-bed, catalytic reactor for the selective oxidation of carbon monoxide can be controlled for a broad range of operating conditions, flow rates, by controlling critical input parameters of the gas stream entering the reactor.
In the present application, an adiabatic reactor is defined as a reactor having no active heat removal device but which may have normal cooling losses associated typically with such reactors. In such a reactor, a temperature rise is observed typically along the length of the reactor for exothermic reactions, such as CO oxidation.
It has been found that in an adiabatically operated fixed-bed, catalytic reactor that the inlet temperature of the entering gas stream, and space velocity of the gas stream in the reactor are determinative of when and to what degree the reverse water gas shift reaction, if at all, will occur within the catalytic reactor. In essence, when the space velocity is changed due to a change in flow rate, such as during partial load operation, adjustments in the inlet temperature of the gas stream can be used to alter the carbon monoxide formation resulting from the reverse water gas shift reaction. Under some circumstances, it might be possible to eliminate entirely the reverse water gas shift reaction and the CO production therefrom.